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The ProcessJan. 21, 2004

Dave EckhardtDave Eckhardt

Bruce MaggsBruce Maggs

L05_Process

15-410“System call abuse for fun & profit”

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Synchronization

Project 0 due at midnightProject 0 due at midnight Please go through the hand-in page now

Anybody reading comp.risks?Anybody reading comp.risks?

TodayToday– Chapter 4, but not exactly!

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Outline

Process as pseudo-machineProcess as pseudo-machine– (that's all there is)

Process life cycleProcess life cycle

Process kernel statesProcess kernel states

Process kernel stateProcess kernel state

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The Computer

Stack

Program

Registers

Keyboard

Screen

Timer

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The Process

Stack

CodeDataHeap

Registers

stdin

stdout

timer

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Process life cycle

BirthBirth– (or, well, fission)

SchoolSchool

WorkWork

DeathDeath

(Nomenclature courtesy of The Godfathers)(Nomenclature courtesy of The Godfathers)

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Birth

Where do new processes come from?Where do new processes come from?– (Not: under a cabbage leaf, by stork, ...)

What do we need?What do we need?– Memory contents

● Text, data, stack– CPU register contents (N of them)– "I/O ports"

● File descriptors, e.g., stdin/stdout/stderr– Hidden “stuff”

● timer state, current directory, umask

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Birth

Intimidating?Intimidating?

How to specify all of that stuff?How to specify all of that stuff?– What is your {name,quest,favorite_color}?

Gee, we already have Gee, we already have oneone process we like... process we like...

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Birth – fork() - 1

MemoryMemory– Copy all of it– Maybe using VM tricks so it' s cheaper

RegistersRegisters– Copy all of them

● All but one: parent learns child's process ID, child gets 0

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Birth – fork() - 2

File descriptorsFile descriptors– Copy all of them– Can't copy the files!– Copy references to open-file state

Hidden stuffHidden stuff– Do whatever is "obvious"

ResultResult– Original, “parent”, process– Fully-specified “child” process, with 0 fork() parameters

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Now what?

Two copies of the same process is Two copies of the same process is boringboring

Transplant surgery!Transplant surgery!– Implant new memory!

● New program text– Implant new registers!

● Old ones don't point well into the new memory– Keep (most) file descriptors

● Good for cooperation/delegation– Hidden state?

● Do what's “obvious”

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Original Process

Stack

/bin/shDataHeap

Registers

stdin

stdout

timer t=4

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Toss Heap, Data

Stack

/bin/sh

Registers

stdin

stdout

timer t=4

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Load New Code, Data From File

Stack

/u/b/gccData

Registers

stdin

stdout

timer t=4

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Reset Stack, Heap

Stack

/u/b/gccData

Registers

stdin

stdout

timer t=4[Heap]

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Fix “Stuff”

Stack

/u/b/gccData

Registers

stdin

stdout

timer off[Heap]

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Initialize Registers

Stack

/u/b/gccData

Registers

stdin

stdout

timer off[Heap]

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Begin Execution

Stack

/u/b/gccData

Registers

stdin

stdout

timer offHeap

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What's This Procedure Called?

int execve( char *path, char *argv[ ], char *envp[ ])

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Birth - other ways

There is another wayThere is another way– Well, two

spawn()spawn()– Carefully specify all features of new process– Don't need to copy stuff you will immediately toss

Plan 9 rfork() / Linux clone()Plan 9 rfork() / Linux clone()– Build new process from old one– Specify which things get shared vs. copied

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School

Old process calledOld process called

execve(char *path,char *argv[ ],char

*envp[ ]);

Result isResult ischar **environ;main(int argc, char *argv[ ]){ ...}

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School

How does the magic work?How does the magic work?– 15-410 motto: No magic

Kernel process setup: we saw...Kernel process setup: we saw...– Toss old data memory– Toss old stack memory– Load executable file

Also...Also...

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The Stack!

Kernel builds stack for new processKernel builds stack for new process– Transfers argv[] and envp[] to top of new process stack– Hand-crafts stack frame for __main()– Sets registers

● Stack pointer (to top frame)● Program counter (to start of __main())

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Work

Process statesProcess states– Running

● User mode● Kernel mode

– Runnable● User mode● Kernel mode

– Sleeping● In condition_wait(), more or less

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Work

Other process statesOther process states– Forking– Zombie

““Exercise for the reader”Exercise for the reader”– Draw the state transition diagram

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Death

VoluntaryVoluntary

void exit(int reason);

Software exceptionSoftware exception– SIGXCPU – used "too much" CPU time

Hardware exceptionHardware exception– SIGSEGV - no memory there for you!

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Death

kill(pid, sig);kill(pid, sig);

^C kill(getpid(), SIGINT);

Start loggingStart logging

kill(daemon_pid, SIGUSR1);% kill -USR1 33

Lost in SpaceLost in Spacekill(Will_Robinson, SIGDANGER);

– I apologize to IBM for lampooning their serious signal● No, I apologize for that apology...

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Process cleanup

Resource releaseResource release– Open files: close()

● TCP: 2 minutes (or more)● Solaris disk offline - forever (“None shall pass!”)

– Memory: release

AccountingAccounting– Record resource usage in a magic file

Gone?Gone?

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“All You Zombies...”

Zombie processZombie process– Process state reduced to exit code– Wait around until parent calls wait()

● Copy exit code to parent memory● Delete PCB

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Kernel process state

The dreaded "PCB"The dreaded "PCB"– (polychlorinated biphenol?)

Process Control BlockProcess Control Block– “Everything without a memory address”

● Kernel management information● Scheduler state● The “stuff”

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Sample PCB contents

CPU register save areaCPU register save area

Process number, parent process numberProcess number, parent process number

Countdown timer valueCountdown timer value

Memory segment infoMemory segment info– User memory segment list– Kernel stack reference

Scheduler infoScheduler info– linked list slot, priority, “sleep channel”

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Conceptual Memory Layout

Stack

Program

k-stackk-stack

k-stackk-stack

Kernel Data

Kernel Program

Stack

Program

Stack

Program

Stack

Program

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Physical Memory Layout

Kernel Memory

User Memory

16 MB

256 MB

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Ready to Implement All This?

Not so complicated...Not so complicated...– getpid()– fork()– exec()– wait()– exit()

What could possibly go wrong?What could possibly go wrong?

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Summary

Parts of a ProcessParts of a Process Virtual – Memory regions, registers, I/O “ports” Physical – Memory pages, registers, I/O devices

Birth, School, Work, DeathBirth, School, Work, Death

““Big Picture” of memory – both of themBig Picture” of memory – both of them (Numbers & arrangement are 15-410–specific) (Numbers & arrangement are 15-410–specific)